Next Steps in Signaling H. Schulzrinne
Internet-Draft Columbia U.
Expires: November 28, 2004 R. Hancock
Siemens/RMR
May 30, 2004
GIMPS: General Internet Messaging Protocol for Signalingdraft-ietf-nsis-ntlp-02
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Abstract
This document specifies protocol stacks for the routing and transport
of per-flow signaling messages along the path taken by that flow
through the network. The design uses existing transport and security
protocols under a common messaging layer, the General Internet
Messaging Protocol for Signaling (GIMPS), which provides a universal
service for diverse signaling applications. GIMPS does not handle
signaling application state itself, but manages its own internal
state and the configuration of the underlying transport and security
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Internet-Draft GIMPS May 20041. Introduction
Signaling involves the manipulation of state held in network
elements. 'Manipulation' could mean setting up, modifying and
tearing down state; or it could simply mean the monitoring of state
which is managed by other mechanisms.
This specification concentrates specifically on the case of
"path-coupled" signaling, which involves network elements which are
located on the path taken by a particular data flow, possibly
including but not limited to the flow endpoints. Indeed, there are
almost always more than two participants in a path-coupled-signaling
session, although there is no need for every router on the path to
participate. Path-coupled signaling thus excludes end-to-end
higher-layer application signaling (except as a degenerate case) such
as ISUP (telephony signaling for Signaling System #7) messages being
transported by SCTP between two nodes.
In the context of path-coupled signaling, examples of state
management include network resource allocation (for "resource
reservation"), firewall configuration, and state used in active
networking; examples of state monitoring are the discovery of
instantaneous path properties (such as available bandwidth, or
cumulative queuing delay). Each of these different uses of
path-coupled signaling is referred to as a signaling application.
Every signaling application requires a set of state management rules,
as well as protocol support to exchange messages along the data path.
Several aspects of this support are common to all or a large number
of applications, and hence should be developed as a common protocol.
The framework given in [22] provides a rationale for a function split
between the common and application specific protocols, and gives
outline requirements for the former, the 'NSIS Transport Layer
Protocol' (NTLP).
This specification provides a concrete solution for the NTLP. It is
based on the use of existing transport and security protocols under a
common messaging layer, the General Internet Messaging Protocol for
Signaling (GIMPS). Different signaling applications may make use of
different services provided by GIMPS, but GIMPS does not handle
signaling application state itself; in that crucial respect, it
differs from application signaling protocols such as the control
component of FTP, SIP and RTSP. Instead, GIMPS manages its own
internal state and the configuration of the underlying transport and
security protocols to ensure the transfer of signaling messages on
behalf of signaling applications in both directions along the flow
path.
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Internet-Draft GIMPS May 20041.1 Restrictions on Scope
This section briefly lists some important restrictions on GIMPS
applicability and functionality. In some cases, these are implicit
consequences of the functionality splits developed in the framework;
in others, they are restrictions on the types of scenario in which
GIMPS can operate correctly.
Flow splitting: In some cases, e.g. where packet-level load sharing
has been implemented, the path taken by a single flow in the
network may not be well defined. If this is the case, GIMPS
cannot route signaling meaningfully. (In some circumstances,
GIMPS can detect this condition, but this cannot be guaranteed.)
Multicast: GIMPS does not handle multicast flows. This includes
'classical' IP multicast and any of the 'small group multicast'
schemes recently proposed.
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Internet-Draft GIMPS May 20042. Requirements Notation and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2].
The terminology used in this specification is fully defined in this
section. The basic entities relevant at the GIMPS level are shown in
Figure 1.
GIMPS (adjacent) peer nodes
IP addresses = Signaling
IP address Source/Destination Addresses IP address
= Flow (depending on signaling direction) = Flow
Source | | Destination
Address | | Address
V V
+--------+ Data +------+ +------+ +--------+
| Flow |-----------|------|-------------|------|-------->| Flow |
| Sender | Flow | | | | |Receiver|
+--------+ |GIMPS |============>|GIMPS | +--------+
| Node |<============| Node |
+------+ Signaling +------+
GN1 Flow GN2
>>>>>>>>>>>>>>>>> = Downstream direction
<<<<<<<<<<<<<<<<< = Upstream direction
Figure 1: Basic Terminology
[Data] Flow: A set of packets identified by some fixed combination of
header fields. Flows are unidirectional (a bidirectional
communication is considered a pair of unidirectional flows).
Session: A single application layer flow of information for which
some network control state information is to be manipulated or
monitored. IP mobility may cause the mapping between sessions and
flows to change, and IP multihoming may mean there is more than
one flow for a given session.
[Flow] Sender: The node in the network which is the source of the
packets in a flow. Could be a host or a router (if the flow is
actually an aggregate).
[Flow] Receiver: The node in the network which is the sink for the
packets in a flow.
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Downstream: In the same direction as the data flow.
Upstream: In the opposite direction to the data flow.
GIMPS Node: Any node along the data path supporting GIMPS (regardless
of what signaling applications it supports).
Adjacent peer: The next GIMPS node along the data path, in the
upstream or downstream direction. Whether two nodes are adjacent
is determined implicitly by the GIMPS peer discovery mechanisms;
it is possible for adjacencies to 'skip over' intermediate GIMPS
nodes if they have no interest in the signaling messages being
exchanged.
Datagram mode: A mode of sending GIMPS messages between nodes without
using any transport layer state or security protection. Upstream
messages are sent UDP encapsulated directly to the signaling
destination; downstream messages are sent towards the flow
receiver with a router alert option.
Connection mode: A mode of sending GIMPS messages directly between
nodes using point to point "messaging associations" (see below),
i.e. transport protocols and security associations.
Messaging association: A single connection between two explicitly
identified GIMPS adjacent peers, i.e. between a given signaling
source and destination address. A messaging association uses a
specific transport protocol and known ports, and may be run over
specific network layer security associations, or use a transport
layer security association internally. A messaging association is
bidirectional.
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Internet-Draft GIMPS May 20043. Design Methodology3.1 Overall Approach
The generic requirements identified in [22] for transport of
path-coupled signaling messages are essentially two-fold:
"Routing": Determine how to reach the adjacent signaling node along
the data path (the GIMPS peer);
"Transport": Deliver the signaling information to that peer.
To meet the routing requirement, for downstream signaling the node
can either use local state information (e.g. gathered during
previous signaling exchanges) to determine the identity of the GIMPS
peer explicitly, or it can just send the signaling towards the flow
destination address and rely on the peer to intercept it. For
upstream signaling, only the first technique is possible.
Once the routing decision has been made, the node has to select a
mechanism for transport of the message to the peer. GIMPS divides
the transport problems into two categories, the easy and the
difficult ones. It handles the easy cases within GIMPS itself,
avoiding complexity and latency, while drawing on the services of
well-understood reliable transport protocols for the harder cases.
Here, with details discussed later, "easy" messages are those that
are sized well below the lowest MTU along a path, are infrequent
enough not to cause concerns about congestion and flow control, and
do not need transport or network-layer security protection.
However, in many cases, signaling information needs to be delivered
between GIMPS peers with additional transport or security properties.
For example, signaling applications could implement their own
reliability mechanism, but experience with RSVP has shown [14] that
relying solely on soft-state refreshes may yield unsatisfactory
performance if signaling messages are lost even occasionally. The
provision of this type of reliability is therefore also the
responsibility of the underlying transport protocols.
In [22] all of these routing and transport requirements are assigned
to a single notional protocol, the 'NSIS Transport Layer Protocol'
(NTLP). The strategy of splitting the transport problem leads to a
layered structure for the NTLP, as a specialised GIMPS 'messaging'
layer running over standard transport and security protocols, as
shown in Figure 2.
GIMPS offers two modes of transport operation:
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The datagram mode uses an unreliable unsecured datagram transport
mechanism, with UDP as the initial choice. The connection mode can
use any stream or message-oriented transport protocol. It may employ
specific network layer security associations (e.g. IPsec), or an
internal transport layer security association (e.g. TLS).
It is possible to mix these two modes along a chain of nodes, without
coordination or manual configuration. This allows, for example, the
use of datagram mode at the edges of the network and connection mode
in the core of the network. Such combinations may make operation
more efficient for mobile endpoints, while allowing multiplexing of
signaling messages across shared security associations and transport
connections between core routers.
It must be understood that the routing and transport decisions made
by GIMPS are not totally independent. If the message transfer has
requirements that enforce the use of connection mode (e.g. because
fragmentation is required), this can only be used between explicitly
identified nodes. In such cases, the GIMPS node must carry out
signaling in datagram mode to identify the peer and then set up the
necessary transport connection. The datagram mode option of sending
the message in the direction of the flow receiver and relying on
interception is not available.
In general, the state associated with connection mode messaging to a
particular peer (signaling destination address, protocol and port
numbers, internal protocol configuration and state information) is
referred to as a "messaging association". There may be any number of
messaging associations between two GIMPS peers (although the usual
case is 0 or 1), and they are set up and torn down by management
actions within GIMPS itself.
3.2 Design Attributes
Soft state: All parts of GIMPS state are subject to time-out
("soft-state"). 'State' here includes the messaging associations
managed by GIMPS.
Application-neutral: GIMPS is designed to support the largest range
of signaling applications. While a number of such applications
have been identified, it appears likely that new ones will emerge.
Mobility support: End systems can change their network attachment
point and network address during a session. GIMPS minimises the
use of IP addresses as identifiers for non-topological information
(e.g. authentication state).
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Efficient: Signaling often occurs before an application such as an IP
telephone conversation can commence, so that any signaling delay
becomes noticeable to the application. Signaling delays are
incurred by the delay in finding signaling nodes along the path
(peer discovery), in retransmitting lost signaling messages and in
setting up security associations between nodes, among other
factors. GIMPS attempts to minimise these delays by several
mechanisms, such as the use of high performance transport
protocols to circumvent message loss, and the re-use of messaging
associations to avoid setup latency. If explicit discovery is
needed it is a lightweight process which only probes local
topology, and GIMPS also allows it to be bypassed completely for
downstream datagram mode messages.
IP version neutral: GIMPS supports both IPv4 and IPv6: it can use
either for transport, largely as a result of their support in the
underlying transport protocols, and can signal for either type of
flow. In addition, GIMPS is able to operate on dual-stack nodes
(to bridge between v4 and v6 regions) and also to operate across
v4/v6 boundaries and other addressing boundaries. Specific
transition issues are considered in Section 6.5.
Transport neutral: GIMPS can operate over any message or
stream-oriented transport layer, including UDP, DCCP, TCP and
SCTP. Messages sent over protocols that do not offer a native
fragmentation service, such as UDP or DCCP, are strictly limited
in size to avoid loss-amplification; in the case of UDP, they must
also be limited in rate to avoid network congestion.
Proxy support: The end systems in a session may not be capable of
handling either the signaling transport or the application and may
instead rely on proxies to initiate and terminate signaling
sessions. Proxy support is limited to nodes that are actually on
the data path (for example, access routers for stub networks);
signaling from a 3rd party node not associated with the data path
is not considered. GIMPS decouples the operation of the messaging
functions from the flow source and destination addresses, treating
these primarily as data.
Scaleable: As will be discussed in Section 4.3, up to one messaging
association is generally kept for each adjacent GIMPS peer and
thus association state scales better than the number of sessions.
(Many peers may not have association state at all, if there are no
messages for sessions visiting those nodes that warrant such
treatment.) Messaging associations are managed based on policy at
each node, depending on trade-offs between fast peer-to-peer
communication and state overhead. Messaging association state can
be removed immediately after the last signaling session to a
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particular next-hop is removed, after some delay to wait for new
sessions, or only if resource demands warrant it.
3.3 Example of Operation
This section presents an example of GIMPS usage in a relatively
simple (in particular, NAT-free) signaling scenario, to illustrate
its main features.
Consider the case of an RSVP-like signaling application which
allocates resources for a flow from sender to receiver; we will
consider how GIMPS transfers messages between two adjacent peers
along the path, GN1 and GN2 (see Figure 1). In this example, the
end-to-end exchange is initiated by the signaling application
instance in the sender; we take up the story at the point where the
first message is being processed (above the GIMPS layer) by the
signaling application in GN1.
1. The signaling application in GN1 determines that this message is
a simple description of resources that would be appropriate for
the flow. It determines that it has no special security or
transport requirements for the message, but simply that it should
be transferred to the next downstream signaling application peer
on the path that the flow will take.
2. The message payload is passed to the GIMPS layer in GN1, along
with a definition of the flow and description of the transfer
requirements {downstream, unsecured, unreliable}. GIMPS
determines that this particular message does not require
fragmentation and that it has no knowledge of the next peer for
this flow and signaling application; however, it also determines
that this application is likely to require secured upstream and
downstream transport of large messages in the future. This
determination is a function of node-local policy, and some
options for how it may be communicated between NSLP and GIMPS
implementations within a node are indicated in Appendix D.
3. GN1 therefore constructs a UDP datagram with the signaling
application payload, and additional payloads at the GIMPS level
to be used to initiate the possible setup of a messaging
association. This datagram is injected into the network,
addressed towards the flow destination and with a Router Alert
Option included.
4. This D-mode message passes through the network towards the flow
receiver, and is seen by each router in turn. GIMPS-unaware
routers will not recognise the RAO value and will forward the
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message unchanged; GIMPS-aware routers which do not support the
signaling application in question will also forward the message
unchanged, although they may need to process more of the message
to decide this.
5. The message is finally intercepted at GN2. The GIMPS layer
identifies that the message is relevant to a local signaling
application, and passes the signaling application payload and
flow description to upwards to it. From there, the signaling
application in GN2 can continue to process this message as in GN1
(compare step 1), and this will eventually result in the message
reaching the flow receiver.
6. In parallel, the GIMPS instance in GN2 recognises that GN1 is
attempting to discover GN2 in order to set up a messaging
association for future signaling for the flow. There are two
possible cases:
A. GN1 and GN2 already have an appropriate association. GN2
simply records the identity of GN1 as its upstream peer for
that flow and signaling application, and sends a GIMPS
message back to GN1 over the association identifying itself
as the peer for this flow.
B. No messaging association exists. Again, GN2 records the
identity of GN1 as before, but sends an upstream D-mode
message to GN1, identifying itself and agreeing to the
association setup. The protocol exchanges needed to complete
this will proceed in the background, controlled by GN1.
7. Eventually, another signaling application message works its way
upstream from the receiver to GN2. This message contains a
description of the actual resources requested, along with
authorisation and other security information. The signaling
application in GN2 passes this payload to the GIMPS level, along
with the flow definition and transfer requirements {upstream,
secured, reliable}.
8. The GIMPS layer in GN2 identifies the upstream peer for this flow
and signaling application as GN1, and determines that it has a
messaging association with the appropriate properties. The
message is queued on the association for transmission (this may
mean some delay if the negotiations begun in step 6.B have not
yet completed).
Further messages can be passed in each direction in the same way.
The GIMPS layer in each node can in parallel carry out maintenance
operations such as route change detection (this can be done by
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sending additional GIMPS-only datagram mode messages, see Section 6.1
for more details).
It should be understood that many of these details of GIMPS
operations can be varied, either by local policy or according to
signaling application requirements, and they are also subject to
development and refinement as the protocol design proceeds. The
authoritative details are contained in the remainder of this
document.
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Internet-Draft GIMPS May 20044. GIMPS Processing Overview
This section defines the basic structure and operation of GIMPS. It
is divided into three parts. Section 4.1 gives an overview of the
per-flow and per-peer state that GIMPS maintains for the purpose of
transferring messages. Section 4.2 describes how messages are
processed in the case where any necessary messaging associations and
associated routing state already exist; this includes the simple
scenario of pure datagram mode operation, where no messaging
associations are necessary in the first place (equivalent to the
transport functionality of base RSVP as defined in [9]). Section 4.3
describes how routing state is maintained and how messaging
associations are initiated and terminated.
4.1 GIMPS State4.1.1 Message Routing State
For each flow, the GIMPS layer can maintain message routing state to
manage the processing of outgoing messages. This state is
conceptually organised into a table with the following structure.
The primary key (index) for the table is the combination of the
information about how the message is to be routed, the session being
signalled for, and the signaling application itself:
Message Routing Information (MRI): This defines the method to be used
to route the message, and any associated addressing information.
In the simplest case, the message routing method is to follow the
path that is being taken by the data flow, and the associated
addressing is the flow header N-tuple (i.e. the Flow-Identifier
of [22]).
Session Identification (SID): This is a cryptographically random and
(probabilistically) globally unique identifier of the application
layer session that is using the flow. For a given flow, different
signaling applications may or may not use the same session
identifier. Often there will only be one flow for a given
session, but in mobility/multihoming scenarios there may be more
than one and they may be differently routed.
Signaling Application Identification (NSLPID): This is an IANA
assigned identifier of the signaling application which is
generating messages for this flow. The inclusion of this
identifier allows the routing state to be different for different
signaling applications (e.g. because of different adjacencies).
The state information for a given key is as follows:
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Upstream peer: the adjacent GIMPS peer closer to the flow source.
This could be an IP address (learned from previous signaling) or a
pointer to a messaging association. It could also be null, if
this node is the flow sender or this node is not storing reverse
routing state, or a special value to indicate that this node is
the last upstream node (but not the sender).
Downstream peer: the adjacent GIMPS peer closer to the flow
destination. This could be a pointer to a messaging association,
or it could be null, if this node is the flow receiver or this
node is only sending downstream datagram mode messages for this
flow and signaling application, or a special value to indicate
that this node is the last downstream node (but not the receiver).
Note that both the upstream and downstream peer state may be null,
and that the session identifier information is not actually required
for message processing; in that case, no state information at all
needs to be stored in the table. Both items of peer identification
state have associated timers for how long the identification can be
considered accurate; when these timers expire, the peer
identification (IP address or messaging association pointer) is
purged if it has not been refreshed. An example of a routing state
table for a simple scenario is given in Appendix B.
Note also that the information is described as a table of flows, but
that there is no implied constraint on how the information is stored.
For example, in a network using pure destination address routing
(without load sharing or any form of policy-based forwarding), the
downstream peer information might be possible to store in an
aggregated form in the same manner as the IP forwarding table. In
addition, many of the per-flow entries may point to the same per-peer
state (e.g. the same messaging association) if the flows go through
the same adjacent peer. However, in general, and especially if GIMPS
peers are several IP hops away, there is no way to identify the
correct downstream peer for a flow and signaling application from the
local forwarding table using prefix matching, and the same applies
always to upstream peer state because of the possibility of
asymmetric routing. Per-flow routing state has to be stored, just as
for RSVP [9].
4.1.2 Messaging Association State
The per-flow message routing state is not the only state stored by
GIMPS. There is also the state required to manage the messaging
associations. Since we assume that these associations are typically
per-peer rather than per-flow, they are stored in a separate table,
including the following information:
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+--x--u-----------d--+ +--d-----------u--x--+
| IP Layer | | IP Layer |
| (Receive Side) | | (Transmit Side) |
+--x--u-----------d--+ +--d-----------u--x--+
x u d d u x
x u d d u x
x u d d u x
uuuuuuuuuuuuuu = upstream datagram mode messages
dddddddddddddd = downstream datagram mode messages
xxxxxxxxxxxxxx = connection mode messages
RAO = Router Alert Option
Figure 3: Message Paths through a GIMPS Node
Note that the same messages are used for maintaining internal GIMPS
state and carrying signaling application payloads. The state
maintenance takes place as a result of processing specific GIMPS
payloads in these messages. The processing of these payloads is the
subject of Section 4.3.
4.2.1 Message Reception
Messages can be received in connection or datagram mode, and from
upstream or downstream peers.
Reception in connection mode is simple: incoming packets undergo the
security and transport treatment associated with the messaging
association, and the messaging association provides complete messages
to the GIMPS layer for further processing. Unless the message is
protected by a query/response cookie exchange (see Section 4.3, the
routing state table is checked to ensure that this messaging
association is associated with the MRI/SID/NSLPID combination.
Reception in datagram mode depends on the message direction.
Upstream messages (from a downstream peer) will arrive UDP
encapsulated and addressed directly to the receiving signaling node.
Each datagram contains a single complete message which is passed to
the GIMPS layer for further processing, just as in the connection
mode case.
Downstream datagram mode messages are UDP encapsulated with an IP
router alert option to cause interception. The signaling node will
therefore 'see' all such messages. The case where the NSLPID does
not match a local signaling application is considered below in
Section 4.2.4; otherwise, it is passed up to the GIMPS layer for
further processing as in the other cases.
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Internet-Draft GIMPS May 20044.2.2 Local Processing
Once a message has been received, by any method, it is processed
locally within the GIMPS layer. The GIMPS processing to be done
depends on the payloads carried; most of the GIMPS-internal payloads
are associated with state maintenance and are covered in Section 4.3.
One GIMPS-internal payload which is carried in each message and
requires processing is the GIMPS hop count. This is decremented on
input processing, and checked to be greater than zero on output
processing. The primary purpose of the GIMPS hop count is to prevent
message looping.
The remainder of the GIMPS message consists of an NSLP payload. This
is delivered locally to the signaling application identified at the
GIMPS level; the format of the NSLP payload is not constrained by
GIMPS, and the content is not interpreted.
Signaling applications can generate their messages for transmission,
either asynchronously, or in response to an input message, and GIMPS
can also generate messages autonomously. Regardless of the source,
outgoing messages are passed downwards for message transmission.
4.2.3 Message Transmission
When a message is available for transmission, GIMPS uses internal
policy and the stored routing state to determine how to handle it.
The following processing applies equally to locally generated
messages and messages forwarded from within the GIMPS or signaling
application levels.
The main decision is whether the message must be sent in connection
mode or datagram mode. Reasons for using the former could be:
o NSLP requirements: for example, the signaling application has
requested channel secured delivery, or reliable delivery;
o protocol specification: for example, this document could specify
that a message that requires fragmentation MUST be sent over a
messaging association;
o local GIMPS policy: for example, a node may prefer to send
messages over a messaging association to benefit from congestion
control.
In principle, as well as determining that some messaging association
must be used, GIMPS could select between a set of alternatives, e.g.
for load sharing or because different messaging associations provide
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different transport or security attributes (see Section 8.5 for
further discussion).
If the use of a messaging association is selected, the message is
queued on the association (found from the upstream or downstream peer
state table), and further output processing is carried out according
to the details of the protocol stack used for the association. If no
appropriate association exists, the message is queued while one is
created (see Section 4.3). If no association can be created, this is
again an error condition, and should be indicated back to the NSLP.
If a messaging association is not required, the message is sent in
datagram mode. The processing in this case depends on whether the
message is directed upstream or downstream.
o If the upstream peer IP address is available from the per-flow
routing table, the message is UDP encapsulated and sent directly
to that address. Otherwise, the message cannot be forwarded (i.e.
this is again an error condition).
o In the downstream direction, messages can always be sent. They
are simply UDP encapsulated and IP addressed using information
from the MRI, with the appropriate router alert option.
4.2.4 Bypass Forwarding
A GIMPS node may have to handle messages for which it has no
signaling application corresponding to the message NSLPID. There are
several possible cases depending mainly on the RAO setting (see
Section 8.4 for more details):
A downstream datagram mode message contains an RAO value associated
with NSIS, and the IP layer is unable to determine whether to forward
it.
A downstream datagram mode message contains an RAO value which is
relevant to the node, but the signaling application for the actual
NSLPID is not processed.
A message is delivered directly (e.g. in C-mode) to the node for
which there is no corresponding signaling application. (According to
the rules of the current specification, this should never happen.
However, future versions might find a use for such a feature.)
In all cases, the role of GIMPS is to forward the message essentially
unchanged. However, a GIMPS implementation must ensure that the IP
TTL field and GIMPS hop count are managed correctly to prevent
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message looping, and this should be done consistently independently
of whether the processing (e.g. for case (1)) takes place on the
fast path or in GIMPS-specific code. The rules are that in cases (1)
and (2), the IP TTL is decremented just as if the message was a
normal IP forwarded packet; in cases (2) and (3) the GIMPS hop count
is decremented as in the case of normal input processing.
4.3 Routing State and Messaging Association Maintenance
The main responsibility of the GIMPS layer is to manage the routing
state and messaging associations which are used in the basic message
processing described above. Routing state is installed and
maintained by datagram mode messages containing specific GIMPS
payloads. Messaging associations are dependent on the existence of
routing state, but are actually set up by the normal procedures of
the transport and security protocols that comprise the messaging
association. Timers control routing state and messaging association
refresh and expiration.
The complete sequence of possible messages for state setup between
adjacent peers is shown in Figure 4 and described in detail in the
following text.
The initial message in any routing state maintenance operation is a
downstream datagram mode message, sent from the querying node and
intercepted at the responding node. This is encapsulated and
addressed just as in the normal case; in particular, it has
addressing and other identifiers appropriate for the flow and
signaling application that state maintenance is being done for, and
it is allowed to contain an NSLP payload. Processing at the querying
and responding nodes is also essentially the same. However, the
querying node includes additional payloads: its own address
information, a proposal for possible messaging association protocol
stacks, and optionally 'Discover-Query' information, including a
Response Request flag and a Query Cookie. This message is informally
referred to as a 'GIMPS-query'. The role of the cookies in this and
subsequent messages is to protect against certain denial of service
attacks.
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already exists towards the querying node, this can be used to deliver
the GIMPS-response message; otherwise, datagram mode is used.
The querying node installs the responder address as downstream peer
state information after verifying the Query Cookie in the
GIMPS-response. The responding node can install the querying address
as upstream peer state information at two points in time:
1. after the receipt of the initial GIMPS-query, or
2. after a third message in the downstream direction containing the
Responder Cookie.
The detailed constraints on precisely when state information is
installed are driven by local policy driven by security
considerations on prevention of denial-of-service attacks and state
poisoning attacks, which are discussed further in Section 7.
Setup of messaging associations begins when both downstream peer
addressing information is available and a new messaging association
is actually needed. (In many cases, the GIMPS-response message above
will identify a downstream peer for whom an appropriate messaging
association already exists, in which case no further action is
needed.) Setup of the messaging association always starts from the
upstream node, but it can be used equally in both directions. The
negotiation of what protocols to use for the messaging association is
controlled by the Stack Proposal information exchanged, and the
processing is outline in Section 6.6.
Refresh and expiration of all types of state is controlled by timers.
State in the routing table has a per-flow, per-direction timer, which
expires after a routing state lifetime. It is the responsibility of
the querying node to generate a GIMPS-query message, optionally with
a Discover-Query payload, before this timer expires, if it believes
that the flow is still active. Receipt of the message at the
responding node will refresh upstream peer addressing state, and
receipt of a GIMPS-response at the querying node will refresh any
downstream peer addressing state if it exists. Note that nodes do
not control the refresh of upstream peer state themselves, they are
dependent on the upstream peer for this.
Messaging associations can be managed by either end. Management
consists of tearing down unneeded associations. Whether an
association is needed is a local policy decision, which could take
into account the cost of keeping the messaging association open, the
level of past activity on the association, and the likelihood of
future activity (e.g. if there are flows still in place which might
generate messages that would use it). Messaging associations can
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always be set up on demand, and messaging association status is not
made directly visible outside the GIMPS layer. Therefore, even if
GIMPS tears down and later re-establishes a messaging association,
signaling applications cannot distinguish this from the case where
the association is kept permanently open.
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Internet-Draft GIMPS May 20045. Message Formats and Encapsulations5.1 GIMPS Messages
All GIMPS messages begin with a common header, which includes a
version number, information about message type, signaling
application, and additional control information. The remainder of
the message is encoded in an RSVP-style format, i.e., as a sequence
of type-length-value (TLV) objects. This subsection describes the
possible GIMPS messages and their contents at a high level; a more
detailed description of each information element is given in Section5.2.
The following gives the syntax of GIMPS messages in ABNF [3].
GIMPS-message: A message is either a datagram mode message or a
connection mode message. GIMPS can detect which by the encapsulation
the message arrives over.
GIMPS-message = D-message / C-message
D-message: A datagram mode message is either upstream or downstream
(slightly different contents are allowed); the common header contains
a flag to say which.
D-message = D-upstream-message / D-downstream-message
C-message: A connection mode message is either upstream or downstream
(again, slightly different contents are allowed); the common header
contains a flag to say which. Note that upstream and downstream
messages can be mixed on a single messaging association.
C-message = C-upstream-message / C-downstream-message
D-downstream-message: A downstream datagram mode message is used for
the GIMPS-query and final handshake in the discovery procedure, and
can also be used simply for carrying NSLP data. Note that the
Common-Header includes a flag to indicate whether an explicit
response is required.
D-downstream-message = Common-Header
Message-Routing-Information
Node-Addressing
Session-Identification
[ Stack-Proposal ]
[ Query-Cookie / Responder-Cookie ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
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D-upstream-message: An upstream datagram mode message is used for the
GIMPS-response in the discovery procedure, and can also be used
simply for carrying NSLP data.
D-upstream-message = Common-Header
Message-Routing-Information
Node-Addressing
Session-Identification
[ Stack-Proposal ]
[ Query-Cookie [ Responder-Cookie ] ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
C-downstream-message: A downstream connection mode message is used
primarily for carrying NSLP data, but can also be used for the final
handshake during discovery. Connection mode messages do not carry
node addressing, since this can be inferred from the messaging
association.
C-downstream-message = Common-Header
Message-Routing-Information
Session-Identification
[ Responder-Cookie ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
C-upstream-message: An upstream connection mode message is used
primarily for carrying NSLP data, but can also be used for the
GIMPS-response during discovery (which is the only case where a
Stack-Proposal TLC can be included).
C-upstream-message = Common-Header
Message-Routing-Information
Session-Identification
[ Stack-Proposal ]
[ Query-Cookie [ Responder-Cookie ] ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
5.2 Information Elements
This section describes the content of the various information
elements that can be present in each GIMPS message, both the common
header, and the individual TLVs. The format description in terms of
bit patterns is provided (in an extremely preliminary form) in
Appendix C.
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Internet-Draft GIMPS May 20045.2.1 The Common Header
Each message begins with a fixed format common header, which contains
the following information:
Version: The version number of the GIMPS protocol.
Length: The number of TLVs following in this message.
Signaling application identifier (NSLPID): This describes the
specific signaling application, such as resource reservation or
firewall control.
GIMPS hop counter: A hop counter to prevent a message from looping
indefinitely.
U/D flag: A bit to indicate if this message is to propagate upstream
or downstream relative to the flow.
Response requested flag: A bit to indicate that this message contains
a cookie which must be echoed in the response.
5.2.2 TLV Objects
All data following the common header is encoded as a sequence of
type-length-value objects. Currently, each object can occur at most
once; the set of required and permitted objects is determined by the
message type and further information in the common header.
These items are contained in each GIMPS message:
Message-Routing-Information (MRI): Information sufficient to define
the route that the flow will take through the network.
Message-Routing-Information = message-routing-method
method-specific-information
The format of the method-specific-information depends on the
message-routing-method requested by the signaling application. In
the basic path-coupled case, it is just the Flow Identifier as in
[22]. Minimally, this could just be the flow destination address;
however, to account for policy based forwarding and other issues a
more complete set of header fields should be used (see Section 6.2
and Section 6.3 for further discussion).
Flow-Identifier = network-layer-version
source-address prefix-length
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destination-address prefix-length
IP-protocol
traffic-class
[ flow-label ]
[ ipsec-SPI / L4-ports]
Additional control information defines whether the flow-label, SPI
and port information are present, and whether the IP-protocol and
traffic-class fields should be interpreted as significant.
Session-Identification (SID): The GIMPS session identifier is a long,
cryptographically random identifier chosen by the node which
begins the signaling exchange (the signaling application at the
node may specify it explicitly, or leave it up to GIMPS to select
a value). The length is open, but 128 bits should be more than
sufficient to make the probability of collisions orders of
magnitude lower than other failure reasons. The session
identifier should be considered immutable end-to-end along the
flow path (GIMPS never changes it, and signaling applications
should propagate it unchanged on messages for the same session).
The following items are optional:
Node addressing: Minimally, this is the IP address at which the GIMPS
node originating the message can be reached; this will be used to
fill in peer routing state. It may also include a logical
interface identifier to assist in route change handling, see
Section 6.1, and port and other information relevant to the
messaging association protocols. This field must be considered
mutable to allow for NAT traversal. The level of flexibility
required in this field is discussed in Section 8.5.
Stack Proposal: This field contains information about which
combinations of transport and security protocols are proposed for
use in messaging associations. This field must be considered
immutable between GIMPS peers; see Section 6.6 for further
details.
Query-Cookie/Responder-Cookie: A query-cookie is optional in a
GIMPS-query message and if present must be echoed in a
GIMPS-response; a response-cookie is optional in a GIMPS-response
message, and if present must be echoed in the following downstream
message. Cookies are variable length (chosen by the cookie
generator) and need to be designed so that a node can determine
the validity of a cookie without keeping state. A future version
of this specification will include references to techniques for
generating such cookies.
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Routing-State-Lifetime: The lifetime of GIMPS routing state in the
absence of refreshes, measured in seconds. Defaults to 30
seconds.
NSLP-Data: The NSLP payload to be delivered to the signaling
application. GIMPS does not interpret the payload content.
5.3 Encapsulation in Datagram Mode
Encapsulation in datagram mode is simple. The complete set of GIMPS
payloads for a single message is concatenated together with the
common header, and placed in the data field of a UDP datagram. UDP
checksums should be enabled. Upstream messages are directly
addressed to the adjacent peer. Downstream messages are addressed
using information from the Message-Routing-Information and
encapsulated with a Router Alert Option. Open issues about
alternative encapsulations, addressing possibilities, and router
alert option value-field setting are discussed in Section 8.2,
Section 8.3 and Section 8.4 respectively.
The source UDP port is selected by the message sender. A destination
UDP port should be allocated by IANA. Note that GIMPS may send
messages addressed as {flow sender, flow receiver} which could make
their way to the flow receiver even if that receiver were
GIMPS-unaware. This should be rejected (with an ICMP message) rather
than delivered to the user application (which would be unable to use
the source address to identify it as not being part of the normal
data flow). Therefore, a "well-known" port would seem to be
required.
For the case of basic path-coupled signaling where the MRI
information is the Flow Identifier, it is vital that the D-mode
message truly mimics the actual data flow, since this is the basis of
how the signaling message is attached to the data path. To this end,
GIMPS may set the traffic class and (for IPv6) flow label to match
the values in the Flow-Identifier if this would be needed to ensure
correct routing. Similar considerations may apply to other message
routing methods if defined.
5.4 Encapsulation in Connection Mode
Encapsulation in connection mode is more complex, because of the
variation in available transport functionality. This issue is
treated in Section 5.4.1. The actual encapsulation is given in
Section 5.4.2.
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Internet-Draft GIMPS May 20045.4.1 Choice of Transport Protocol
It is a general requirement of the NTLP defined in [22] that it
should be able to support bundling (of small messages), fragmentation
(of large messages), and message boundary delineation. Not all
transport protocols natively support all these features.
SCTP [6] satisfies all requirements. (The bundling requirement is
met implicitly by the use of Nagle-like algorithms inside the SCTP
stack.)
DCCP [7] is message based but does not provide bundling or
fragmentation. Bundling can be carried out by the GIMPS layer
sending multiple messages in a single datagram; because the common
header includes length information (number of TLVs), the message
boundaries within the datagram can be discovered during parsing.
Fragmentation of GIMPS messages over multiple datagrams should be
avoided, because of amplification of message loss rates that this
would cause.
TCP provides both bundling and fragmentation, but not message
boundaries. However, the length information in the common header
allows the message boundary to be discovered during parsing.
UDP can be augmented as in the DCCP case. (An additional reason for
avoiding fragmentation is the lack of congestion control
functionality in UDP.)
It can be seen that all of these protocol options can be supported by
the basic GIMPS message format already presented. GIMPS messages
requiring fragmentation must be carried using a reliable transport
protocol, TCP or SCTP.
5.4.2 Encapsulation Format
The GIMPS message, consisting of common header and TLVs, is carried
directly in the transport protocol (possibly incorporating transport
layer security protection). Further GIMPS messages can be carried in
a continuous stream (for TCP), or up to the next transport layer
message boundary (for SCTP/DCCP/UDP). This situation is shown in
Figure 5; it applies to both upstream and downstream messages.
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Internet-Draft GIMPS May 20046. Advanced Protocol Features6.1 Route Changes and Local Repair6.1.1 Introduction
When re-routing takes place in the network, GIMPS and signaling
application state needs to be updated for all flows whose paths have
changed. The updates to signaling application state are usually
signaling application dependent: for example, if the path
characteristics have actually changed, simply moving state from the
old to the new path is not sufficient. Therefore, GIMPS cannot carry
out the complete path update processing. Its responsibilities are to
detect the route change, update its own routing state consistently,
and inform interested signaling applications at affected nodes.
Route change management is complicated by the distributed nature of
the problem. Consider the re-routing event shown in Figure 6. An
external observer can tell that the main responsibility for
controlling the updates will probably lie with nodes A and E;
however, D1 is best placed to detect the event quickly at the GIMPS
level, and B1 and C1 could also attempt to initiate the repair.
On the assumption that NSLPs are soft-state based and operate end to
end, and because GIMPS also periodically updates its picture of
routing state, route changes will eventually be repaired
automatically. However, especially if NSLP refresh times are
extended to reduce signaling load, the duration of inconsistent state
may be very long indeed. Therefore, GIMPS includes logic to deliver
prompt notifications to NSLPs, to allow NSLPs to carry out local
repair if possible.
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o Detecting that the upstream path has (or may have) changed (in
which case the node may no longer be on the path at all).
At a single node, these processes are largely independent, although
clearly a change in downstream path at a node corresponds to a change
in upstream path at the downstream peer. Note that there are two
possible aspects of route change:
Interface: The interface through which a flow leaves or enters a node
may change.
Peer: The adjacent upstream peer or downstream peer may change.
In general, a route change could include one or the other or both.
(In theory it could include neither, although such changes are hard
to detect and even harder to do anything useful about.)
There are five mechanisms for a GIMPS node to detect that a route
change has occurred, which are listed below. They apply differently
depending on whether the change is in the upstream or downstream
path, and these differences are summarised in the following table.
Local Trigger: In trigger mode, a node finds out that the next hop
has changed. This is the RSVP trigger mechanism where some form
of notification mechanism from the routing table to the protocol
handler is assumed. Clearly this only works if the routing change
is local, not if the routing change happens somewhere a few
routing hops away (including the case that the change happens at a
GIMPS-unaware node).
Extended Trigger: An extended trigger, where the node checks a
link-state routing table to discover that the path has changed.
This makes certain assumptions on consistency of route computation
(but you probably need to make those to avoid routing loops) and
only works within a single area for OSPF and similar link-state
protocols. Where available, this offers the most accurate and
expeditious indication of route changes, but requires more access
to the routing internals than a typical OS may provide.
GIMPS C-mode Monitoring: A node may find that C-mode packets are
arriving (from upstream or downstream peer) with a different TTL
or on a different interface. This provides no direct information
about the new flow path, but indicates that routing has changed
and that rediscovery may be required.
Data Plane Monitoring: The signaling application on a node may detect
a change in behaviour of the flow, such as TTL change, arrival on
a different interface, or loss of the flow altogether. The
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signaling application on the node is allowed to notify this
information locally to GIMPS.
GIMPS D-mode Probing: In probing mode, each GIMPS node periodically
repeats the discovery (GIMPS-query/GIMPS-response) operation. The
querying node will discover the route change by a modification in
the Node-Addressing information in the GIMPS-response. This is
similar to RSVP behavior, except that there is an extra degree of
freedom since not every message needs to repeat the discovery,
depending on the likely stability of routes. All indications are
that, leaving mobility aside, routes are stable for hours and
days, so this may not be necessary on a 30-second interval,
especially if the other techniques listed above are available.
When these methods discover a route change in the upstream direction,
this cannot be handled directly by GIMPS at the detecting node, since
route discovery proceeds only in the downstream direction.
Therefore, to exploit these mechanisms, it must be possible for GIMPS
to send a notification message in the upstream direction to initiate
this. (This would be possible for example by setting an additional
flag in the Common-Header of an upstream message.)
+----------------------+----------------------+---------------------+
| Method | Downstream | Upstream |
+----------------------+----------------------+---------------------+
| Local Trigger | Discovers new | Not applicable |
| | downstream interface | |
| | (and peer if local) | |
| | | |
| Extended Trigger | Discovers new | May determine that |
| | downstream interface | route from upstream |
| | and may determine | peer will have |
| | new downstream peer | changed |
| | | |
| C-Mode Monitoring | Provides hint that | Provides hint that |
| | change has occurred | change has occurred |
| | | |
| Data Plane | Not applicable | NSLP informs GIMPS |
| Monitoring | | that a change may |
| | | have occurred |
| | | |
| D-Mode Probing | Discovers changed | Discovers changed |
| | Node-Addressing in | Node-Addressing in |
| | GIMPS-response | GIMPS-query |
+----------------------+----------------------+---------------------+
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Internet-Draft GIMPS May 20046.1.3 Local Repair
Once a node has detected that a change may have occurred, there are
three possible cases:
1. Only an upstream change is indicated. There is nothing that can
be done locally; GIMPS must propagate a notification to its
upstream peer.
2. A downstream change has been detected and an upstream change
cannot be ruled out. Although some local repair may be
appropriate, it is difficult to decide what, since the path
change may actually have taken place upstream of the detecting
node (so that this node is no longer on the path at all).
3. A downstream change has been detected, but there is no upstream
change. In this case, the detecting node is the true crossover
router, i.e. the point in the network where old and new paths
diverge. It is the correct node to initiate the local repair
process.
In case (3), i.e. at the upstream crossover node, the local repair
process is initiated by the GIMPS level as follows:
o GIMPS marks its downstream routing state information for this flow
as 'invalid', unless the route change was actually detected by
D-mode probing (in which case the new state has already been
installed).
o GIMPS notifies the local NSLP that local repair is necessary.
It is assumed that the second step will typically trigger the NSLP to
generate a downstream message, and the attempt to send it will
stimulate a GIMPS-query/response. This signaling application message
will propagate downstream, also discovering the new route, until it
rejoins the old path; the node where this happens may also have to
carry out local repair actions.
A problem is that there is usually no robust technique to distinguish
case (2) from case (3), because of the relative weakness of the
techniques in determining that upstream change has not occurred.
(They can be effective in determining that a change has occurred;
however, even where they can tell that the route from the upstream
peer has not changed, they cannot rule out a change beyond that
peer.) There is therefore a danger that multiple nodes within the
network would attempt to carry out local repair in parallel.
One possible technique to address this problem is that a GIMPS node
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that detects case (3) locally, rather than initiating local repair
immediately, still sends a route change notification upstream, just
in case (2) actually applies. If the upstream peer locally detects
no downstream route change, it can signal this to the downstream node
(e.g. by setting another flag in the Common-Header of a GIMPS
message). This acts to damp the possibility of a 'local repair
storm', at the cost of an additional peer-peer round trip time.
6.1.4 Local Signaling Application State Removal
After a route change, a signaling application may wish to remove
state at another node which is no longer on the path. However, since
it is no longer on the path, in principle GIMPS can no longer send
messages to it. (In general, provided this state is soft, it will
time out anyway; however, the timeouts involved may have been set to
be very long to reduce signaling load.) The requirement to remove
state in a specific peer node is identified in [25].
This requirement can be met provided that GIMPS is able to 'remember'
the old path to the signaling application peer for the period while
the NSLP wishes to be able to use it. Since NSLP peers are a single
GIMPS hop apart, the necessary information is just the old entry in
the node's routing state table for that flow. Rather than requiring
the GIMPS level to maintain multiple generations of this information,
it can just be provided to the signaling application in the same node
(in an opaque form), which can store it if necessary and provide it
back to the GIMPS layer in case it needs to be used. This
information is denoted as 'SII-Handle' in the abstract API of
Appendix D; however, the details are an implementation issue which do
not affect the rest of the protocol.
6.1.5 Operation with Heterogeneous NSLPs
A potential problem with route change detection is that the detecting
GIMPS node may not implement all the signaling applications that need
to be informed. Therefore, it would need to be able to send a
notification back along the unchanged path to trigger the nearest
signaling application aware node to take action. If multiple
signaling applications are in use, it would be hard to define when to
stop propagating this notification. However, given the rules on
message interception and routing state maintenance in Section 4.2,
Section 4.3 and Section 8.4, this situation cannot arise: all NSLP
peers are exactly one GIMPS hop apart.
The converse problem is that the ability of GIMPS to detect route
changes by purely local monitoring of forwarding tables is more
limited. (This is probably an appropriate limitation of GIMPS
functionality. If we need a protocol for distributing notifications
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about local changes in forwarding table state, a flow signaling
protocol is probably not the right starting point.)
6.2 Policy-Based Forwarding and Flow Wildcarding
Signaling messages almost by definition need to contain address and
port information to identify the flow they are signaling for. We can
divide this information into two categories:
Message-Routing-Information: This is the information needed to
determine how a message is routed within the network. It may
include a number of flow N-tuple parameters, and is carried as an
object in each GIMPS message (see Section 5.1).
Additional Packet Classification Information: This is any further
higher layer information needed to select a subset of packets for
special treatment by the signaling application. The need for this
is highly signaling application specific, and so this information
is invisible to GIMPS (if indeed it exists); it will be carried
only in the corresponding NSLP.
The correct pinning of signaling messages to the data path depends on
how well the downstream messages in datagram mode can be made to be
routed correctly. Two strategies are used:
The messages themselves match the flow in destination address and
possibly other fields (see Section 5.3 and Section 8.3 for further
discussion). In many cases, this will cause the messages to be
routed correctly even by GIMPS-unaware nodes.
A GIMPS-aware node carrying out policy based forwarding on higher
layer identifiers (in particular, the protocol and port numbers
for IPv4) should take into account the entire
Message-Routing-Information object in selecting the outgoing
interface rather than relying on the IP layer.
The current Message-Routing-Information format allows a limited
degree of 'wildcarding', for example by applying a prefix length to
the source or destination address, or by leaving certain fields
unspecified. A GIMPS-aware node must verify that all flows matching
the Message-Routing-Information would be routed identically in the
downstream direction, or else reject the message with an error.
6.3 NAT Traversal
As already noted, GIMPS messages must carry packet addressing and
higher layer information as payload data in order to define the flow
signalled for. (This applies to all GIMPS messages, regardless of
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how they are encapsulated or which direction they are travelling in.)
At an addressing boundary the data flow packets will have their
headers translated; if the signaling payloads are not likewise
translated, the signaling messages will refer to incorrect (and
probably meaningless) flows after passing through the boundary.
The simplest solution to this problem is to require that a NAT is
GIMPS-aware, and to allow it to modify datagram mode messages based
on the contents of the Message-Routing-Information payload. (This is
making the implicit assumption that NATs only rewrite the header
fields included in this payload, and not higher layer identifiers.)
Provided this is done consistently with the data flow header
translation, signaling messages will be valid each side of the
boundary, without requiring the NAT to be signaling application
aware. An outline of the set of operations necessary on a downstream
datagram mode message is as follows:
1. Verify that bindings for the data flow are actually in place.
2. Create bindings for subsequent C-mode signaling (based on the
information in the Node-Addressing field).
3. Create a new Message-Routing-Information payload with fields
modified according to the data flow bindings.
4. Create a new Node-Addressing payload with fields to force
upstream D-mode messages through the NAT, and to allow C-mode
exchanges using the C-mode signaling bindings.
5. Forward the message with these new payloads.
The original Message-Routing-Information and Node-Addressing payloads
should be retained in the message, but encapsulated in a new TLV
type. (In the case of a sequence of NATs, this TLV would become a
list.) This TLV essentially becomes a recorded route for the D-mode
message; a GIMPS node that wished to do topology hiding could replace
these original payloads with opaque tokens, or omit them altogether.
Note that a consequence of this approach is that the routing state
tables at the actual signaling application peers (either side of the
NAT) are no longer directly compatible (in particular, the values of
Message-Routing-Information are different.
The case of traversing a GIMPS unaware NAT is for further study.
There is a dual problem of whether the GIMPS peers either side of the
boundary can work out how to address each other, and whether they can
work out what translation to apply to the Message-Routing-Information
from what is done to the signaling packet headers. The fundamental
problem is that GIMPS messages contain 3 or 4 interdependent
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addresses which all have to be consistently translated, and existing
generic NAT traversal techniques such as STUN [21] can process only
two.
6.4 Interaction with IP Tunnelling
The interaction between GIMPS and IP tunnelling is very simple. An
IP packet carrying a GIMPS message is treated exactly the same as any
other packet with the same source and destination addresses: in other
words, it is given the tunnel encapsulation and forwarded with the
other data packets.
Tunnelled packets will not be identifiable as GIMPS messages until
they leave the tunnel, since any router alert option and the standard
GIMPS protocol encapsulation (e.g. port numbers) will be hidden
behind the standard tunnel header. If signaling is needed for the
tunnel itself, this has to be initiated as a separate signaling
session by one of the tunnel endpoints - that is, the tunnel counts
as a new flow. Because the relationship between signaling for the
'microflow' and signaling for the tunnel as a whole will depend on
the signaling application in question, we are assuming that it is a
signaling application responsibility to be aware of the fact that
tunnelling is taking place and to carry out additional signaling if
necessary; in other words, one tunnel endpoint must be signaling
application aware.
In some cases, it is the tunnel exit point (i.e. the node where
tunnelled data and downstream signaling packets leave the tunnel)
that will wish to carry out the tunnel signaling, but this node will
not have knowledge or control of how the tunnel entry point is
carrying out the data flow encapsulation. This information could be
carried as additional data (an additional GIMPS payload) in the
tunnelled signaling packets if the tunnel entry point was at least
GIMPS aware. This payload would be the GIMPS equivalent of the RSVP
SESSION_ASSOC object of [12]. Whether this functionality should
really be part of GIMPS and if so how the payload should be handled
will be considered in a later version.
6.5 IPv4-IPv6 Transition and Interworking
GIMPS itself is essentially IP version neutral (version dependencies
are isolated in the formats of the Message-Routing-Information and
Node-Addressing TLVs, and GIMPS also depends on the version
independence of the protocols that support messaging associations).
In mixed environments, GIMPS operation will be influenced by the IP
transition mechanisms in use. This section provides a high level
overview of how GIMPS is affected, considering only the currently
predominant mechanisms.
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Dual Stack: (This applies both to the basic approach described in
[26] as well as the dual-stack aspects of more complete
architectures such as [28].) In mixed environments, GIMPS should
use the same IP version as the flow it is signaling for; hosts
which are dual stack for applications and routers which are dual
stack for forwarding should have GIMPS implementations which can
support both IP versions.
In theory, for some connection mode encapsulation options, a
single messaging association could carry signaling messages for
flows of both IP versions, but the saving seems of limited value.
The IP version used in datagram mode is closely tied to the IP
version used by the data flow, so it is intrinsically impossible
for a IPv4-only or IPv6-only GIMPS node to support signaling for
flows using the other IP version.
Applications with a choice of IP versions might select a version
for which GIMPS support was available in the network, which could
be established by running parallel discovery procedures. In
theory, a GIMPS message related to a flow of one IP version could
flag support for the other; however, given that IPv4 and IPv6
could easily be separately routed, the correct GIMPS peer for a
given flow might well depend on IP version anyway, making this
flagged information irrelevant.
Packet Translation: (Applicable to SIIT [5] and NAT-PT [13].) Some
transition mechanisms allow IPv4 and IPv6 nodes to communicate by
placing packet translators between them. From the GIMPS
perspective, this should be treated essentially the same way as
any other NAT operation (e.g. between 'public' and 'private'
addresses) as described in Section 6.3. In other words, the
translating node needs to be GIMPS aware; it will run GIMPS with
IPv4 on some interfaces and with IPv6 on others, and will have to
translate the Message-Routing-Information payload between IPv4 and
IPv6 formats for flows which cross between the two. The
translation rules for the fields in the payload (including e.g.
traffic class and flow label) are as defined in [5].
Tunnelling: (Applicable to 6to4 [15] and a whole host of other
tunnelling schemes.) Many transition mechanisms handle the problem
of how an end to end IPv6 (or IPv4) flow can be carried over
intermediate IPv4 (or IPv6) regions by tunnelling; the methods
tend to focus on minimising the tunnel administration overhead.
From the GIMPS perspective, the treatment should be as similar as
possible to any other IP tunnelling mechanism, as described in
Section 6.4. In particular, the end to end flow signaling will
pass transparently through the tunnel, and signaling for the
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tunnel itself will have to be managed by the tunnel endpoints.
However, additional considerations may arise because of special
features of the tunnel management procedures. For example, [16]
is based on using an anycast address as the destination tunnel
endpoint. It might be unwise to carry out signaling for the
tunnel to such an address, and the GIMPS implementation there
would not be able to use it as a source address for its own
signaling messages (e.g. GIMPS-responses). Further analysis will
be contained in a future version of this specification.
6.6 Messaging Association Protocol Negotiation
A key attribute of GIMPS is that it is flexible in its ability to use
existing transport and security protocols. Different transport
protocols may have performance attributes appropriate to different
environments; different security protocols may fit appropriately with
different authentication infrastructures. Even if a single choice
for GIMPS could be agreed today, the need to support new protocols in
the future cannot be ruled out. Therefore, some sort of protocol
negotiation capability is required.
The implicit requirements for protocol negotiation are as follows:
o It should be possible to request a set of protocols (e.g. TLS/TCP
or SCTP/IPsec), not just a single protocol.
o The negotiation should complete in 1 RTT.
o The negotiation should be resistant to bidding-down ("man in the
middle") attacks.
o At the same time, the message elements involved should allow NAT
traversal.
o The set of possible protocols should be extensible.
The stacking requirements are reminiscent of [10], and the
negotiation requirements of [20], and the following outline is based
on the same principles. In particular, the latter should be read for
a more detailed security discussion.
o Each possible "protocol layer" is represented by an IANA-assigned
tag. A protocol layer defines a well-known protocol (such as
"TCP") and a set of rules for its use (such as "Connect from
Querying Node").
o A protocol layer may define some security related parameters, and
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will probably also define some addressing options; the latter
would be carried in the Node-Addressing TLV and are not considered
further here.
o A "profile" is a sequence of protocol layers.
o A Stack-Proposal TLV consists of a sequence of profiles, and any
associated security parameters.
o When attempting to set up a messaging association, a node includes
a Stack-Proposal TLV in the GIMPS-query. The contents of the TLV
must be fixed for a given outbound interface and NSLPID.
o The responding node includes another Stack-Proposal in the
GIMPS-Response. The contents of this TLV must also be fixed for a
given outbound interface and NSLPID.
o The querying node selects a common profile from the proposals and
sets up the protocol layers accordingly. Once the messaging
association is open, it repeats the Stack-Proposal from the
GIMPS-Response. The responding node can verify this to ensure
that no bidding down attack has occurred.
The exchanges parallel the cookie exchanges which protect routing
state setup, but they are largely independent. (The cookie exchanges
can be used to protect nodes from denial of service attacks
masquerading as a messaging association protocol setup, in case the
connection setup procedure for one of the protocols is
DoS-vulnerable, as is the case with TCP.)
It is expected that the initial set of protocol layers will be very
small; however, it could be extended, e.g. to allow for different
configurations (e.g. "Connect from Responding Node" or requiring the
use of particular protocol options) or entirely new protocols.
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Internet-Draft GIMPS May 20047. Security Considerations
The security requirement for the GIMPS layer is to protect the
signaling plane against identified security threats. For the
signaling problem as a whole, these threats have been outlined in
[23]; the NSIS framework [22] assigns a subset of the responsibility
to the NTLP. The main issues to be handled can be summarised as:
Message Protection: Signaling message content should be protected
against eavesdropping, modification, injection and replay while in
transit. This applies both to GIMPS payloads, and GIMPS should
also provide such protection as a service to signaling
applications between adjacent peers.
State Integrity Protection: It is important that signaling messages
are delivered to the correct nodes, and nowhere else. Here,
'correct' is defined as 'the appropriate nodes for the signaling
given the Message-Routing-Information'. In the case where the MRI
is the Flow Identification for path-coupled signalling,
'appropriate' means 'the same nodes that the infrastructure will
route data flow packets through'. (GIMPS has no role in deciding
whether the data flow itself is being routed correctly; all it can
do is ensure the signaling is routed consistently with it.) GIMPS
uses internal state to decide how to route signaling messages, and
this state needs to be protected against corruption.
Prevention of Denial of Service Attacks: GIMPS nodes and the network
have finite resources (state storage, processing power,
bandwidth). The protocol should try to minimise exhaustion
attacks against these resources and not allow GIMPS nodes to be
used to launch attacks on other network elements.
The main missing issue is handling authorisation for executing
signaling operations (e.g. allocating resources). This is assumed
to be done in each signaling application.
In many cases, GIMPS relies on the security mechanisms available in
messaging associations to handle these issues, rather than
introducing new security measures. Obviously, this requires the
interaction of these mechanisms with the rest of the GIMPS protocol
to be understood and verified, and some aspects of this are discussed
in Section 6.6.
7.1 Message Confidentiality and Integrity
GIMPS can use messaging association functionality, such as TLS or
IPsec, to ensure message confidentiality and integrity. In many
cases, confidentiality of GIMPS information itself is not likely to
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be a prime concern, in particular since messages are often sent to
parties which are unknown ahead of time, although the content visible
even at the GIMPS level gives significant opportunities for traffic
analysis. Signaling applications may have their own mechanism for
securing content as necessary; however, they may find it convenient
to rely on protection provided by messaging associations,
particularly if this is provided efficiently and if it runs unbroken
between signaling application peers.
7.2 Peer Node Authentication
Cryptographic protection (of confidentiality or integrity) requires a
security association with session keys, which can be established
during an authentication and key exchange protocol run based on
shared secrets, public key techniques or a combination of both.
Authentication and key agreement is possible using the protocols
associated with the messaging association being secured (TLS
incorporates this functionality directly; IKE, IKEv2 or KINK can
provide it for IPsec). GIMPS nodes rely on these protocols to
authenticate the identity of the next hop, and GIMPS has no
authentication capability of its own.
However, with discovery, there are few effective ways to know what is
the legitimate next or previous hop as opposed to an impostor. In
other words, cryptographic authentication here only provides
assurance that a node is 'who' it is (i.e. the legitimate owner of
identity in some namespace), not 'what' it is (i.e. a node which is
genuinely on the flow path and therefore can carry out signaling for
a particular flow). Authentication provides only limited protection,
in that a known peer is unlikely to lie about its role. Additional
methods of protection against this type of attack are considered in
Section 7.3 below.
It is open whether peer node authentication should be made signaling
application dependent; for example, whether successful authentication
could be made dependent on presenting authorisation to act in a
particular signaling role (e.g. signaling for QoS). The abstract
API of Appendix D allows GIMPS to forward such policy and
authentication decisions to the NSLP it is serving.
7.3 Routing State Integrity
The internal state in a node (see Section 4.1), specifically the
upstream and downstream peer identification, is used to route
messages. If this state is corrupted, signaling messages may be
misdirected.
In the case where the message routing method is path-coupled
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signaling, the messages need to be routed identically to the data
flow described by the Flow Identifier, and the routing state table is
the GIMPS view of how these flows are being routed through the
network in the immediate neighbourhood of the node. Routes are only
weakly secured (e.g. there is usually no cryptographic binding of a
flow to a route), and there is no other authoritative information
about flow routes than the current state of the network itself.
Therefore, consistency between GIMPS and network routing state has to
be ensured by directly interacting with the routing mechanisms to
ensure that the upstream and downstream signaling peers are the
appropriate ones for any given flow. A good overview of security
issues and techniques in this sort of context is provided in [27].
Downstream peer identification is installed and refreshed only on
receiving a GIMPS-reponse message (compare Figure 4). This must echo
the cookie from a previous GIMPS-query message, which will have been
sent downstream along the flow path (in datagram mode, i.e.
end-to-end addressed). Hence, only the true next peer or an on-path
attacker will be able to generate such a message, provided freshness
of the cookie can be checked at the querying node.
Upstream peer identification can be installed directly on receiving a
GIMPS-query message containing addressing information for the
upstream peer. However, any node in the network could generate such
a message (indeed, almost any node in the network could be the
genuine upstream peer for a given flow). To protect against this,
two strategies are possible:
Filtering: the receiving node may be able to reject signaling
messages which claim to be for flows with flow source addresses
which would be ruled out by ingress filtering. An extension of
this technique would be for the receiving node to monitor the data
plane and to check explicitly that the flow packets are arriving
over the same interface and if possible from the same link layer
neighbour as the datagram mode signaling packets. (If they are
not, it is likely that at least one of the signaling or flow
packets is being spoofed.) Signaling applications should only
install state on the route taken by the signaling itself.
Authentication (weak or strong): the receiving node may refuse to
install upstream state until it has handshaked by some means with
the upstream peer. This handshaking could be as simple as
requesting the upstream peer to echo the response cookie in the
discover-response payload of a GIMPS-response message (to
discourage nodes impersonating upstream peers from using forged
source addresses); or, it could be full peer authentication within
the messaging association, the reasoning being that an
authenticated peer can be trusted not to pretend that it is on
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path when it is not.
The second technique also plays a role in denial of service
prevention, see below. In practice, a combination of both techniques
may be appropriate.
7.4 Denial of Service Prevention
GIMPS is designed so that each connectionless discovery message only
generates at most one response, so that a GIMPS node cannot become
the source of a denial of service amplification attack.
However, GIMPS can still be subjected to denial-of-service attacks
where an attacker using forged source addresses forces a node to
establish state without return routability, causing a problem similar
to TCP SYN flood attacks. In addition to vulnerabilities of a next
peer discovery an unprotected path discovery procedure might
introduce more denial of service attacks since a number of nodes
could possibly be forced to allocate state. Furthermore, an
adversary might modify or replay unprotected signaling messages.
There are two types of state attacks and one computational resource
attack. In the first state attack, an attacker floods a node with
messages that the node has to store until it can determine the next
hop. If the destination address is chosen so that there is no
GIMPS-capable next hop, the node would accumulate messages for
several seconds until the discovery retransmission attempt times out.
The second type of state-based attack causes GIMPS state to be
established by bogus messages. A related computational/
network-resource attack uses unverified messages to cause a node to
make AAA queries or attempt to cryptographically verify a digital
signature. (RSVP is vulnerable to this type of attack.) Relying only
on upper layer security, for example based on CMS, might open a
larger door for denial of service attacks since the messages are
often only one-shot-messages without utilizing multiple roundtrips
and DoS protection mechanisms.
There are at least three defenses against these attacks:
1. The responding node does not establish a session or discover its
next hop on receiving the GIMPS-query message, but can wait for a
setup message on a reliable channel. If the reliable channel
exists, the additional delay is a one one-way delay and the total
is no more than the minimal theoretically possible delay of a
three-way handshake, i.e., 1.5 node-to-node round-trip times.
The delay gets significantly larger if a new connection needs to
be established first.
2. The response to the initial discovery message contains a cookie.
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The previous hop repeats the discovery with the cookie included.
State is only established for messages that contain a valid
cookie. The setup delay is also 1.5 round-trip times. (This
mechanism is similar to that in SCTP [6] and other modern
protocols.)
3. If there is a chance that the next-hop node shares a secret with
the previous hop, the sender could include a hash of the session
ID and the sender's secret. The receiver can then verify that
the message was likely sent by the purported source. This does
not scale well, but may work if most nodes tend to communicate
with a small peer clique of nodes. (In that case, however, they
might as well establish more-or-less permanent transport sessions
with each other.)
These techniques are complementary; we chose a combination of the
first and second method.
Once a node has decided to establish routing state, there may still
be transport and security state to be established between peers.
This state setup is also vulnerable to additional denial of service
attacks. GIMPS relies on the lower layer protocols that make up
messaging associations to mitigate such attacks. The current
description assumes that the upstream node is always the one wishing
to establish a messaging association, so it is typically the
downstream node that needs to be protected. Extensions are
considered in Section 8.6; these would require further analysis.
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Internet-Draft GIMPS May 20048. Open Issues8.1 Protocol Naming
Alternate names:
GIST: General Internet Signaling Transport
GIMPS: General Internet Messaging Protocol for Signaling
LUMPS: Lightweight Universal Messaging for Path associated Signaling
There is a danger of some ambiguity as to whether the protocol name
refers to the complete transport stack below the signaling
applications, or only to the additional protocol functionality above
the standard transport protocols (UDP, TCP etc.) The NSIS framework
uses the term NTLP for the first, but this specification uses the
GIST/variants names for the second (see Figure 2 in Section 3.1). In
other words, this specification proposes to meet the requirements for
NTLP functionality by layering GIMPS/... over existing standard
transport protocols. It isn't clear if additional terminological
surgery is needed to make this clearer.
8.2 General IP Layer Issues
Some NSIS messages have to be addressed end-to-end but intercepted at
intermediate routers, and this imposes some special constraints on
how they can be encapsulated. RSVPv1 [9] primarily uses raw IP with
a specific protocol number (46); a UDP encapsulation is also possible
for hosts unable to perform raw network i/o. RSVP aggregation [18]
uses an additional protocol number (134) to bypass certain interior
nodes.
The critical requirements for the encapsulation at this level are
that routers should be able to identify signaling packets for
processing, and that they should not mis-identify packets for
'normal' end-to-end user data flows as signaling packets. The
current assumption is that UDP encapsulation can be used for such
messages, by allocating appropriate (new) value codes for the router
alert option (RAO) [1][4] to identify NSIS messages. Specific open
issues about how to allocate such values are discussed in Section8.4.
An alternative approach would be to use raw IP with the RSVP protocol
numbers and a new RSVP version number. Although this would provide
some more commonality with existing RSVP implementations, the NAT
traversal problems for such an encapsulation seem much harder to
solve. Specifically, any unmodified NAT (which performed address
sharing) would be unable to process any such traffic since they need
to understand a higher-layer field (such as TCP or UDP port) to use
as a demultiplexer.
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Internet-Draft GIMPS May 20048.3 Encapsulation and Addressing for Datagram Mode
The discussion in Section 4 essentially assumes that datagram mode
messages are UDP encapsulated. This leaves open the question of
whether other encapsulations are possible, and exactly how these
messages should be addressed.
As well as UDP/IP (and raw IP as discussed and temporarily ruled out
in Section 8.2), DCCP/IP and UDP/IPsec could also be considered as
'datagram' encapsulations. However, they still require explicit
addressing between GIMPS peer nodes and some per-peer state to be set
up and maintained. Therefore, it seems more appropriate to consider
these encapsulation options as possible messaging association types,
for use where there is a need for congestion control or security
protection but without reliability. This would leave UDP/IP as the
single encapsulation allowed for all datagram mode messages.
Addressing for upstream datagram mode messages is simple: the IP
source address is the signaling source address, and the IP
destination address is the signaling destination address (compare
Figure 1). For downstream datagram mode messages, the IP destination
address will be the flow destination address, but the IP source
address could be either of the flow source address or signaling
source address. Some of the relative merits of these options are as
follows:
o Using the flow source address makes it more likely that the
message will be correctly routed through any intermediate
NSIS-unaware region which is doing load sharing or policy routing
on the {source, destination} address pair. If the signaling
source address is used, the message will be intercepted at some
node closer to the flow destination, but it may not be the same as
the next node for the data flow packets.
o Conversely, using the signaling source address means that ICMP
error messages (specifically, unreachable port or address) will be
correctly delivered to the message originator, rather than being
sent back to the flow source. Without seeing these messages, it
is very difficult for the querying node to recognise that it is
the last NSIS node on the path. In addition, using the signaling
source address may make it possible to exchange messages through
GIMPS unaware NATs (although it isn't clear how useful the
resulting messages will be, see Section 6.3).
It is not clear which of these situations it is more important to
handle correctly and hence which source addressing option to use.
(RSVP uses the flow source address, although this is primarily for
multicast routing reasons.) A conservative approach would be to allow
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both, possibly even in parallel (although this might open up the
protocol to amplification attacks).
8.4 Intermediate Node Bypass and Router Alert Values
We assume that the primary mechanism for intercepting messages is the
use of the RAO. The RAO contains a 16 bit value field, within which
35 values have currently been assigned by IANA. It is open how to
assign values for use by GIMPS messages to optimise protocol
processing, i.e. to minimise the amount of slow-path processing that
nodes have to carry out for messages they are not actually interested
in the content of.
There are two basic reasons why a GIMPS node might wish to ignore a
message:
o because it is for a signaling application that the node does not
process;
o because even though the signaling application is present on the
node, the interface on which the message arrives is only
processing signaling messages at the aggregate level and not for
individual flows (compare [18]).
Conversely, note that a node might wish to process a number of
different signaling applications, either because it was genuinely
multifunctional or because it processed several versions of the same
application. (Note from Appendix C.1 that different versions are
distinguished by different NSLP identifiers.)
Some or all of this information could be encoded in the RAO value
field, which would then allow messages to be filtered on the fast
path. There is a tradeoff between two approaches here, whose
evaluation depends on whether the processing node is specialised or
general purpose:
Fine-Grained: The signaling application (including specific version)
and aggregation level are directly identified in the RAO value. A
specialised node which handles only a single NSLP can efficiently
ignore all other messages; a general purpose node may have to
match the RAO value in a message against a long list of possible
values.
Coarse-Grained: IANA allocates RAO values for 'popular' applications
or groups of applications (such as 'All QoS Signaling
Applications'). This speeds up the processing in a general
purpose node, but a specialised node may have to carry out further
processing on the GIMPS common header to identify the precise
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messages it needs to consider.
These considerations imply that the RAO value should not be tied
directly to the NSLP id, but should be selected for the application
on broader considerations of likely deployment scenarios. Note that
the exact NSLP is given in the GIMPS common header, and some
implementations may still be able to process it on the fast path.
The semantics of the node dropping out of the signaling path are the
same however the filtering is done.
There is a special consideration in the case of the aggregation
level. In this case, whether a message should be processed depends
on the network region it is in (specifically, the link it is on).
There are then two basic possibilities:
1. All routers have essentially the same algorithm for which
messages they process, i.e. all messages at aggregation level 0.
However, messages have their aggregation level incremented on
entry to an aggregation region and decremented on exit.
2. Router interfaces are configured to process messages only above a
certain aggregation level and ignore all others. The aggregation
level of a message is never changed; signaling messages for end
to end flows have level 0, but signaling messages for aggregates
are generated with a higher level.
The first technique requires aggregating/deaggregating routers to be
configured with which of their interfaces lie at which aggregation
level, and also requires consistent message rewriting at these
boundaries. The second technique eliminates the rewriting, but
requires interior routers to be configured also. It is not clear
what the right trade-off between these options is.
8.5 Messaging Association FlexibilitySection 4 discusses the use of 0 or 1 messaging associations between
any pair of GIMPS nodes. However, logically it would be quite
possible to have more than one association, for example:
o to allow different reliability characteristics;
o to provide different levels of security protection or to have
security separation between different signaling streams;
o even simply to have load split between different connections
according to priority (so there could be two associations with
identical protocol stacks).
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It is possible to imagine essentially infinite flexibility in these
options, both in terms of how many possibilities are allowed and how
nodes signal their capabilities and preferences, without much
changing the overall GIMPS structure. (The GIMPS-query and
GIMPS-response messages described in section Section 4.3 can be used
to exchange this information.) What is not clear is how much
flexibility is actually needed.
8.6 Messaging Association Setup Message Sequences
The discussion of Section 4.3 assumes a simple fixed message
sequence, which we can picture as follows:
+---------------------------------+---------------------------------+
| Direction | Message |
+---------------------------------+---------------------------------+
| ---> | GIMPS-query message |
| | |
| <--- | GIMPS-response message |
| | |
| ===> | Querying node initiates |
| | messaging association setup |
| | messages |
| | |
| <--> | Signaling messages exchanged |
+---------------------------------+---------------------------------+
There are several variants which could be considered at this level,
for example whether the messaging association could be set up by the
responding node:
+---------------------------------+---------------------------------+
| Direction | Message |
+---------------------------------+---------------------------------+
| ---> | GIMPS-query message |
| | |
| <=== | Responding node initiates |
| | messaging association setup |
| | messages |
| | |
| <--> | Signaling messages exchanged |
+---------------------------------+---------------------------------+
This saves a message but may be vulnerable to additional denial of
service attacks.
Another open area is how to manage the protocol exchanges that take
place in setting up the messaging association itself. It is probably
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an implementation matter to consider whether to carry out, for
example, the SCTP 4-way handshake only after IKE exchanges (for IPsec
SA initialisation) have completed, or whether these can be done
partly in parallel. A more radical step is to carry the initial
request and response messages of both exchanges as payloads in the
GIMPS-query/response exchange, with the request message initially
formatted by the querying node with an unspecified destination IP
address. This would require modifications to the protocol
implementations (especially at the querying node) similar to what is
needed for NAT traversal; it would have to be evaluated whether this
was worth the one or two round trip times that are saved. Both this
technique and the "reverse connection" approach above can be
considered optimisations; given an appropriate negotiation procedure
in the base protocol as discussed in Section 6.6 they probably do not
need to be considered further for the initial version.
A final area is how the responding node propagates the signaling
message downstream. It could initiate the downstream discovery
process as soon as it received the initial GIMPS-query message, or it
could wait until the first signaling application message has been
received (which might not be until a messaging association has been
established). A similar timing question applies to when it should
initiate its own downstream messaging associations. It is possible
that all these options are simply a matter for implementation
freedom, although leaving them open will make mobility and re-routing
behaviour rather harder to analyse, and again there are denial of
service implications for some approaches (see Section 7.4).
8.7 GIMPS Support for Message Scoping
Many signaling applications are interested in sending messages over a
specific region of the network. Message scoping of this nature seems
to be hard to achieve in a topologically robust way, because such
region boundaries are not well defined in the network layer.
It may be that the GIMPS layer can assist such scoping, by detecting
and counting different types of nodes in the signaling plane. The
simplest solution would be to count GIMPS nodes supporting the
relevant signaling application - this is already effectively done by
the GIMPS hop count. A more sophisticated approach would be to track
the crossing of aggregation region boundaries, as introduced in
Section 8.4. Whether this is plausible depends on the willingness of
operators to configure such boundary information in their routers.
8.8 Additional Discovery Mechanisms
The routing state maintenance procedures described in Section 4.3 are
strongly focussed on the problem of discovering, implicitly or
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explicitly, the neighbouring peers on the flow path - which is the
necessary functionality for path-coupled signaling.
As well as the GIMPS-query/response discovery mechanism, other
techniques may sometimes also be possible. For example, in many
environments, a host has a single access router, i.e. the downstream
peer (for outgoing flows) and the upstream peer (for incoming ones)
are known a priori. More generally, a link state routing protocol
database can be analysed to determine downstream peers in more
complex topologies, and maybe upstream ones if strict ingress
filtering is in effect. More radically, much of the GIMPS protocol
is unchanged if we consider off-path signaling nodes, although there
are significant differences in some of the security analysis (Section7.3). However, none of these possibilities are considered further in
this specification.
8.9 Alternative Message Routing Requirements
The initial assumption of GIMPS is that signaling messages are to be
routed identically to data flow messages. For this case of
path-coupled signaling, the MRI and upstream/downstream flag (in the
Common-Header) define the flow and the relationship of the signaling
to it sufficiently for GIMPS to route its messages correctly.
However, some additional modes of routing signaling messages have
been identified:
Predictive Routing: Here, the intent is to send signaling along a
path that the data flow may or will follow in the future.
Possible cases are pre-installation of state on the backup path
that would be used in the event of a link failure; and predictive
installation of state on the path that will be used after a mobile
node handover. It is currently unclear whether these cases can be
met using the existing GIMPS routing capabilities (and if they
cannot, whether they are in the initial scope of the work).
NAT Address Reservations: This applies to the case where a node
behind a NAT wishes to use NSIS signaling to reserve an address
from which it can be reached by a sender on the other side. This
requires a message to be sent outbound from what will be the flow
receiver although no reverse routing state exists. One possible
solution (assumed in [24]) is to construct a message with the
Flow-Routing-Information matching the possible senders and send it
as though it was downstream signaling. It is not clear whether
signaling for the 'wrong direction' in this way will always be
treated consistently by GIMPS, especially if routing policies and
encapsulations for inbound and outbound traffic are treated very
differently within the rest of the network.
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In the current structure of the protocol definition, the way to
handle these requirements (if they are needed) is to define a new
message routing method which replaces the basic path-coupled version.
The requirements for defining a new routing method include the
following:
o Defining the format of the MRI for the new message routing method
type.
o Defining how D-mode messages should be encapsulated and routed
corresponding to this MRI.
o Defining any filtering or other security mechanisms that should be
used to validate the MRI in a D-mode message.
o Defining how the MRI format is processed on passing through a NAT.
8.10 Congestion Control in Datagram Mode
The GIMPS-query and GIMPS-response messages may suffer from message
loss (e.g. due to congestion or corruption). Because a successful
handshake is necessary before a messaging association can even be
initiated, GIMPS must provide its own recovery method for these
cases. A working assumption is that the querying node can repeat the
GIMPS-query with an exponential backoff until a response is received
or some retry threshold is reached.
More subtle is the case where there is a stream of D-mode messages
with no immediate feedback from the neighbour node. This could be
the case where a signaling application was generating messages for
stateless processing in the interior of the network. Here, the
appropriate approach may be to use rate-limiting algorithms such as
in ICMPv6 [11]. Another possibility would be to use ECN [17], if the
datagram mode messages can be correlated with a congestion controlled
messaging association which also supports ECN. Details are clearly
for further study.
8.11 Message Format Issues
NSIS message formats are defined as a set of objects (see AppendixC.1). Some aspects are left open:
Ordering: Traditionally, Internet protocols require a node to be able
to process a message with objects in any order. However, this has
some costs in parser complexity, testing interoperability, ease of
compression; there is a special issue with GIMPS that for
efficiency, the NTLP-Data object (which may be large) should come
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last. Should object order be fixed or unspecified?
Capabilities: For extensibility, it is useful to be able to mark
objects with some information about how they should be treated if
the receiving node does not implement them (e.g. ignore or
reject). Since the object space is shared between all protocols,
this marking has to be standardised across all the NSIS protocols.
Is an object marking scheme based on some flags in the object
header appropriate, or a more flexible scheme based on some type
of capability encoding?
8.12 Protocol Design Details
Clearly, not all details of GIMPS operation have been defined so far.
This section provides a list of slightly non-trivial areas where more
detail is need, where these have not been mentioned elsewhere in the
text.
o Receiver initiated signaling applications need to have reverse
path state set up in the network, before the signaling application
itself can originate any messages. Should this be done by GIMPS
carrying out the discovery for the specific signaling application
(which requires the flow sender to know what signaling
applications are going to be used), or should the discovery
attempt to find every GIMPS node and the signaling applications
they support?
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Internet-Draft GIMPS May 20049. Change History9.1 Changes In Version -02
Version -02 does not represent any radical change in design or
structure from version -01; the emphasis has been on adding details
in some specific areas and incorporation of comments, including early
review comments. The full list of changes is as follows:
1. Added a new Section 1.1 which summarises restrictions on scope
and applicability; some corresponding changes in terminology in
Section 2.
2. Closed the open issue on including explicit GIMPS state teardown
functionality. On balance, it seems that the difficulty of
specifying this correctly (especially taking account of the
security issues in all scenarios) is not matched by the saving
of state enabled.
3. Removed the option of a special class of message transfer for
reliable delivery of a single message. This can be implemented
(inefficiently) as a degenerate case of C-mode if required.
4. Extended Appendix C with a general discussion of rules for
message and object formats across GIMPS and other NSLPs. Some
remaining open issues are noted in Section 8.11.
5. Updated the discussion of Section 8.4 to take into account the
proposed message formats and rules for allocation of NSLP id,
and propose considerations for allocation of RAO values.
6. Modified the description of the information used to route
messages (first given in Section 4.1.1 but also throughout the
document). Previously this was related directly to the flow
identification and described as the Flow-Routing-Information.
Now, this has been renamed Message-Routing-Information, and
identifies a message routing method and any associated
addressing.
7. Modified the text in Section 4.2 and elsewhere to impose sanity
checks on the Message-Routing-Information carried in C-mode
messages, including the case where these messages are part of a
GIMPS-Query/Response exchange.
8. Added rules for message forwarding to prevent message looping in
a new Section 4.2.4, including rules on IP TTL and GIMPS hop
count processing. These take into account the new RAO
considerations of Section 8.4.
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9. Added an outline mechanism for messaging association protocol
stack negotiation, with the details in a new Section 6.6 and
other changes in Section 4.3 and the various sections on message
formats.
10. Removed the open issue on whether storing reverse routing state
is mandatory or optional. This is now explicit in the API
(under the control of the local NSLP).
11. Added an informative annex describing an abstract API between
GIMPS and NSLPs in Appendix D.
9.2 Changes In Version -01
The major change in version -01 is the elimination of
'intermediaries', i.e. imposing the constraint that signaling
application peers are also GIMPS peers. This has the consequence
that if a signaling application wishes to use two classes of
signaling transport for a given flow, maybe reaching different
subsets of nodes, it must do so by running different signaling
sessions; and it also means that signaling adaptations for passing
through NATs which are not signaling application aware must be
carried out in datagram mode. On the other hand, it allows the
elimination of significant complexity in the connection mode handling
and also various other protocol features (such as general route
recording).
The full set of changes is as follows:
1. Added a worked example in Section 3.3.
2. Stated that nodes which do not implement the signaling
application should bypass the message (Section 4.2).
3. Decoupled the state handling logic for routing state and
messaging association state in Section 4.3. Also, allow
messaging associations to be used immediately in both directions
once they are opened.
4. Added simple ABNF for the various GIMPS message types in a new
Section 5.1, and more details of the common header and each
object in Section 5.2, including bit formats in Appendix C. The
common header format means that the encapsulation is now the
same for all transport types (Section 5.4.1).
5. Added some further details on datagram mode encapsulation in
Section 5.3, including more explanation of why a well known port
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it needed.
6. Removed the possibility for fragmentation over DCCP (Section5.4.1), mainly in the interests of simplicity and loss
amplification.
7. Removed all the tunnel mode encapsulations (old sections 5.3.3
and 5.3.4).
8. Fully re-wrote the route change handling description (Section6.1), including some additional detection mechanisms and more
clearly distinguishing between upstream and downstream route
changes. Included further details on GIMPS/NSLP interactions,
including where notifications are delivered and how local repair
storms could be avoided. Removed old discussion of propagating
notifications through signaling application unaware nodes (since
these are now bypassed automatically). Added discussion on how
to route messages for local state removal on the old path.
9. Revised discussion of policy-based forwarding (Section 6.2) to
account for actual FLow-Routing-Information definition, and also
how wildcarding should be allowed and handled.
10. Removed old route recording section (old Section 6.3).
11. Extended the discussion of NAT handling (Section 6.3) with an
extended outline on processing rules at a GIMPS-aware NAT and a
pointer to implications for C-mode processing and state
management.
12. Clarified the definition of 'correct routing' of signaling
messages in Section 7 and GIMPS role in enforcing this. Also,
opened the possibility that peer node authentication could be
signaling application dependent.
13. Removed old open issues on Connection Mode Encapsulation
(section 8.7); added new open issues on Message Routing (Section8.9) and Datagram Mode congestion control (Section 8.10).
14. Added this change history.
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Internet-Draft GIMPS May 2004Appendix C. Bit-Level Formats
This appendix provides initial formats for the various component
parts of the GIMPS messages defined abstractly in Section 5.2. It
should be noted that these formats are extremely preliminary and
should be expected to change completely several times during the
further development of this specification.
In addition, this appendix includes some general rules for the format
of messages and message objects across all protocols in the NSIS
protocol suite (i.e. the current and future NSLPs as well as GIMPS
itself). The intention of these common rules is to encourage
commonality in implementations, ease of testing and debugging, and
sharing of object definitions across different applications.
C.1 General NSIS Formatting Guidelines
Each NSIS message consists of a header and a sequence of objects. An
NSLP message is one object within a GIMPS message. The GIMPS header
has a specific format, described in more detail in Appendix C.2
below; the NSLP header format is common to all signaling applications
and includes simply a message type (which may be structured into a
type field and some processing flags, depending on the application).
Note that GIMPS provides the message length information and signaling
application identification. There is no version information; if an
NSLP is extended so much that it stops being backwards compatible, a
new signaling application identifier is allocated.
Every object has the following general format:
o The overall format is Type-Length-Value (in that order).
o Assignments for the Type field are common across all NSIS
protocols (i.e. there is a single registry). This is to
facilitate the sharing of common objects across different
signaling applications. How to encode capability information
therefore has to be standardised across signaling applications;
this is still an open issue (see Section 8.11.
o Length has the units of 32 bit words, and measures the length of
Value. If there is no Value, Length=0.
o Value is (therefore) a whole number of 32 bit words. If there is
any padding required, this must be defined by the object-specific
format information; objects which contain variable length (e.g.
string) types may need to include additional length subfields to
do so.
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Internet-Draft GIMPS May 2004Appendix D. API between GIMPS and NSLP
This appendix provides an initial abstract API between GIMPS and
NSLPs.
This does not constrain implementors, but rather helps clarify the
interface between the different layers of the NSIS protocol suite.
In addition, although some of the data types carry the information
from GIMPS Information Elements, this does not imply that the format
of that data as sent over the API is the same.
Conceptually the API has similarities to the UDP sockets API,
particularly that for unconnected UDP sockets. An extension for an
API like that for UDP connected sockets could be considered. In this
case, for example, the only information needed in a SendMessage
primitive would be NSLP-Data, NSLP-Data-Size, and NSLP-Message-Handle
(which can be null). Other information which was persistent for a
group of messages could be configured once for the socket.
D.1 SendMessage
This primitive is passed from an NSLP to GIMPS. It is used whenever
the NSLP wants to send a message.
SendMessage ( NSLP-Data, NSLP-Data-Size, NSLP-Message-Handle,
NSLP-Id, Session-ID,
MRI, Direction, SII-Handle,
Transfer-Attributes, Timeout, IP-TTL )
NSLP-Data: The NSLP message itself.
NSLP-Data-Size: The length of NSLP-Data.
NSLP-Message-Handle: A handle for this message, that can be used
later by GIMPS to reference it in error messages, etc. A NULL
handle may be supplied if the NSLP is not interested in receiving
MessageDeliveryError notifications for this message.
NSLP-Id: An identifier indicating which NSLP this is.
Session-ID: The NSIS session identifier.
MRI: Message routing information for use by GIMPS in determining the
correct next GIMPS hop for this message. It contains, for
example, the flow source/destination addresses and the type of
routing to use for the signalling message.
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Direction: A flag indicating whether the message is to be sent in the
upstream or downstream direction (in relation to the MRI).
SII-Handle: A handle, previously supplied by GIMPS, to a data
structure (identifying peer addresses and interfaces) that should
be used to explicitly route the message to a particular GIMPS next
hop. If supplied, GIMPS should validate that it is consistent
with the MRI.
Transfer-Attributes: Reliability, security, priority etc. attributes
to be used for sending this particular message. A value
indicating "default" or "don't care" may be given.
Timeout: Length of time GIMPS should attempt to send this message
before indicating an error. A value indicating "default" or
"don't care" may be given.
IP-TTL: The value of the IP TTL that should be used when sending this
message. A value indicating "default" or "don't care" may be
given.
D.2 RecvMessage
This primtive is passed from GIMPS to an NSLP. It is used whenever
GIMPS receives a message.
RecvMessage ( [NSLP-Data, NSLP-Data-Size,]
NSLP-Id, Session-ID,
MRI, Direction, SII-Handle,
Transfer-Attributes,
IP-TTL, Original-TTL )
NSLP-Data: The NSLP message itself (may be empty).
NSLP-Data-Size: The length of NSLP-Data (may be zero).
NTLP-Message-Handle: A handle for this message, that can be used
later by the NSLP to reference it in a MessageReceived primitive.
NSLP-Id: An identifier indicating which NSLP this is message is for.
Session-ID: The NSIS session identifier.
MRI: Message routing information that was used by GIMPS in forwarding
this message. It contains, for example, the flow source/
destination addresses and the type of routing to used for the
signalling message.
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Direction: A flag indicating whether the message was received going
in the upstream or downstream direction (in relation to the MRI).
SII-Handle: A handle to a data structure, identifying peer addresses
and interfaces. Can be used to identify route changes and for
explicit routing to a particular GIMPS next hop.
Transfer-Attributes: Reliability, security, priority, etc.
attributes that were used for this particular message.
IP-TTL: The value of the IP TTL (or Hop Limit) associated with this
message.
Original-TTL: The value of the IP TTL (or Hop Limit) at the time of
sending of the packet that contained this message.
D.3 MessageReceived
This primitive is passed from an NSLP to GIMPS. It is used after a
RecvMessage primitive has been passed from GIMPS to an NSLP to inform
GIMPS whether the NSLP wishes GIMPS to retain state.
MessageReceived ( NTLP-Message-Handle, Retain-State )
NTLP-Message-Handle: A handle on a message, previously supplied by
GIMPS in a RecvMessage primitive.
RetainState: A value indicating whether or not the NSLP wishes GIMPS
to retain path state.
D.4 MessageDeliveryError
This primitive is passed from GIMPS to an NSLP. It is used to notify
the NSLP that a message that it requested to be sent has failed to be
dispatched.
MessageDeliveryError ( NSLP-Message-Handle, Error-Type )
NSLP-Message-Handle: A handle for the message provided by the NSLP at
the time of sending.
Error-Type: Indicates the type of error that occurred. For example,
'no next node found'.
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Internet-Draft GIMPS May 2004D.5 NetworkNotification
This primitive is passed from GIMPS to an NSLP. It indicates that a
network event of possible interest to the NSLP occurred.
NetworkNotification ( MRI, Network-Notification-Type )
MRI: Provides the message routing information to which the network
notification applies.
Network-Notification-Type: Indicates the type of event that caused
the notification, e.g. downstream route change, upstream route
change, detection that this is the last node.
D.6 SecurityProtocolAttributesRequest
This primitive is passed from GIMPS to an NSLP. It is sent when
GIMPS requires the NSLP to make decisions (e.g. check policy) or
provide information for authentication parameters to be used when
setting up a messaging association.
SecurityProtocolAttributesRequest ( Peer-Info, Security-Protocol, Request-Type )
Peer-Info: Information identifying the GIMPS peer and interface
Security-Protocol: A value indicating the security protocol being
used (TLS, IPsec, etc).
Request-Type: An indication of the type of information required (e.g.
client certificate)
D.7 SetStateLifetime
This primitive is passed from an NSLP to GIMPS. It indicates the
lifetime for which the NSLP would like GIMPS to retain its state. It
can also give a hint that the NSLP is no longer interested in the
state.
SetStateLifetime ( MRI, Direction, State-Lifetime )
MRI: Provides the message routing information to which the network
notification applies.
Direction: A flag indicating whether this relates to state for the
upstream or downstream direction (in relation to the MRI).
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State-Lifetime: Indicates the lifetime for which the NSLP wishes
GIMPS to retain its state (may be zero, indicating that the NSLP
has no further interest in the GIMPS state).
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